Asthma is a disease that effects about 4% of the population of the United States or about 15 million people. The incidence of asthma has grown at an alarming rate, up nearly 75% in the past decade. It is a very complex disease, having multiple etiologies involving inflammatory cells, mediators, and the cells and tissues that line airways. Symptoms are many and varied including wheezing, shortness of breath, chest tightness and cough. Several classes of medications have been developed to combat asthma such as beta agonists, anti-inflammatory and anti leukotrienes. Aggressive management plans have been designed that include medication, monitoring and lifestyle adjustments to maintain patients at optimal levels of health.
The diagnosis of asthma includes a thorough history and physical exam as well as objective tests of pulmonary function using simple spirometry. Evidence of reversible airway obstruction is a hallmark of the disease. Occasionally patients do not present with airway obstruction at the time of exam. In these cases, asthma is confirmed by a positive response to the inhalation of methacholine, a known asthma provocateur.
Asthma is typically monitored using a simple device to measure Peak Expiratory Flow Rate (PEFR). Peak flow meters are widely used for this purpose. Peak flow meters measure the muscular effort to exhale forcibly from fully inflated lungs as well as the elastic recoil of the lungs and airways function. Thus, peak expiratory flow rate is a simple expression of a complex process of lung emptying.
A wide variety of peak flow meters are commercially available and many more have been patented, but not commercialized. One popular type of peak flow meter is the linear, or “in-line”, peak flow meter. These flow meters typically include a cylindrical housing with an air inlet having an integral air restrictor at one end, an air outlet at the other end, and slot into which an indicator is movably disposed. A piston is disposed within the housing and in contact with a compression spring. When the user blows into the housing, the flow restrictor restricts the flow of air and the piston moves against the force of the air, causing it to move the indicator within the slot as the spring is compressed. When airflow is ceased, the spring forces the piston back to its original position, but the indicator remains at its “peak flow” position within the slot. The position of the indicator is then compared to graduations along the side of the slot to determine the “peak flow” of air from the user.
Although these flow meters are relatively popular, they have a number of drawbacks. First, in order to obtain an acceptable level accuracy, a relatively long spring and housing must be utilized, making the meter cumbersome to transport. Second, the arrangement of the spring, and variations in effective lengths due to manufacturing tolerances, makes these meters difficult to zero. Accordingly, these meters require the use of springs with extremely tight manufacturing tolerances. Third, the location of the air outlet at the end of the meter makes them prone to blockage or influence by the fingers of the user, resulting in corruption of test results. Finally, the location of the air restrictor within the air outlet makes them prone to blockage or influence as well, again resulting in corruption of test results.
A second type of peak flow meter is the “electronic type” of peak flow meter. These meters are typically battery powered and electronically measure the pressure drop across a flow restriction. This peak flow of air from the user is then displayed on a liquid crystal display, or other visual display, and may be stored for later download into a personal computer or other electronic device.
Electronic peak flow meters are relatively accurate and are easy to use. However, they are also generally too large to be truly portable and are much more expensive than non-electronic versions. Further, these units will not operate without properly charged batteries, creating the risk that user will not detect the onset of an asthmatic attack when the batteries are low or completely depleted.
A third type of peak flow meter is the “rotary” peak flow meter. An example of a “rotary” peak flow meter is currently marketed by the assignee of the present invention, Spirometrics, Inc. of Gray, Me., under the trademark “SPIRO-FLOW™”. Rotary peak flow meters typically include a relatively thin rectangular housing having an air inlet at one end, an air outlet having an integral air flow restriction extending from the other, and a semicircular slot disposed along the side of the housing into which an indicator is movably disposed. A vane assembly is rotatably attached to a torsion spring and is dimensioned such that it interferes with the indicator when it is rotated. The torsion spring is fixedly attached to the housing and is oriented to counteract the force of the air on the flag when the user blows into the housing. In operation, the flag is caused to rotate by the force of the air, causing the indicator to move within the slot. When airflow is ceased, the torsion spring causes the flag to rotate back to its original position, but the indicator remains at its “peak flow” position within the slot. The position of the indicator is then compared to graduations along the side of the slot to determine the “peak flow” of air from the user.
Rotary peak flow meters do not require the use of long compression springs to operate and, therefore, can be made small enough to be truly portable. However, conventional rotary peak flow meters are not without drawbacks. First, the accuracy of the measurement is dependent, in part, upon the relative positions of the ends of the torsion spring. Given the tolerances inherent in the process of manufacturing the torsion springs, the variation in this positioning can result in significant inaccuracies. These inaccuracies can be counteracted by customizing each scale for each particular spring. However, this process requires that each unit be tested to generate the values for this custom scale, causing a corresponding increase in cost. Second, like the linear, or “in-line”, peak flow meters, rotary peak flow meters include a single air outlet. As was the case with these meters, this outlet is prone to blockage by the finger of the user, resulting in corruption of test results. Finally, the location of the air flow restriction within the air inlet makes them prone to the same corruption as their linear counterparts.
Therefore, there is a need for a peak flow meter that is small enough to be truly portable, that is sufficiently accurate and repeatable to provide meaningful results, that does not require a customized scale to provide the necessary level of accuracy, that is not prone to corruption of test results through blockage by a user's finger of the air outlet or flow restrictor, that may be manufactured and sold at a relatively low cost, and that does not require the use of batteries.